Call libusb_init on the FILE thread.

device/usb/usb_context.h

Index: device/usb/usb_context.h
diff --git a/device/usb/usb_context.h b/device/usb/usb_context.h
index ee3abfcabfe2def644ad17e0f6f18f1b4c9d4bea..379eee546e94dfeb5c88af23a3621bf7d597c6c2 100644
--- a/device/usb/usb_context.h
+++ b/device/usb/usb_context.h
@@ -23,13 +23,13 @@ typedef libusb_context* PlatformUsbContext;
 // Destructor must be called on FILE thread.
 class UsbContext : public base::RefCountedThreadSafe<UsbContext> {
  public:
+  explicit UsbContext(PlatformUsbContext context);
+
   PlatformUsbContext context() const { return context_; }
 
  protected:
-  friend class UsbServiceImpl;
   friend class base::RefCountedThreadSafe<UsbContext>;
 
-  explicit UsbContext(PlatformUsbContext context);
   virtual ~UsbContext();
 
  private: